Cops who are trying to determine which sister mowed down two women in a weekend hit-and-run in Brooklyn will likely have to test blood found inside the car, of ficials said yesterday.

Days after a car plowed into Erinn Phelan and her friend, Alma Guerrero, and kept going, police have yet to make an arrest.

Detectives thought they had the list of possible drivers narrowed down to one until their hit-and-run suspect pointed the finger at her own sister.

The Acura’s owner, Cindy Jasmin, 31, eluded cops for two days before hiring a lawyer and meeting with detectives.

But the meeting hardly settled the matter after the lawyer told cops that Jasmin’s younger sister, Frances, had been behind the wheel early Sunday morning before checking into a psychiatric center.

The lawyer, Adam Thompson, said Frances was so distraught after the accident that she had to be admitted to a psychiatric ward because she talked of taking her own life.

But police were said to be skeptical of the story.

Jasmin was held briefly in The Bronx on an outstanding bench warrant from last year, when she was caught driving with a suspended license. She was released.

Cops might need scientific evidence to unravel the mystery. Blood found inside the battered car is believed to belong to the driver, who may have been cut when the windshield on the driver’s side was pushed back from the impact.

Cindy Jasmin was identified on a surveillance video at a taxi stand by a man who said she came in saying she had been in an accident. She does not appear to be injured in the video.

Jasmin was holed up in her lawyer’s Manhattan office for much of the day yesterday. Jasmin and Thompson emerged later in the day but declined to comment.

Phelan, 22, a coordinator with Mayor Bloomberg’s volunteer agency, suffered severe head trauma in the accident and was in a coma. Guerrero suffered a broken collarbone.

Witnesses said Phelan pushed Guerrero out of the way, and took the brunt of the impact. The two were roommates at Brown University.

Cops said the driver was driving 42 mph along Flatbush Avenue and had the green light when the two friends were crossing.